Alex Caruso isn’t walking through that door to help the Bulls.

Plus, when would Caruso — the defensive specialist they traded to the Thunder in June for Josh Giddey — ever walk? He’d more likely sprint in, knock the ball from an opponent’s hands, chase it down, dive for it and find an open teammate with a bounce pass while lying on his back.

But still, Caruso is gone. And coach Billy Donovan’s crew needs to start doing something about it.

A little over a week ago, the Bulls (3-6) were seventh in the NBA in defensive efficiency, but even Donovan believed that felt off, given what he has seen from this group.

Sure enough, they’ll enter Saturday’s game against the Hawks ranked 20th — a drop closely tied to their allowing 45 points in the fourth quarter against the Timberwolves on Thursday in a game they had controlled through the first three quarters.

“The start of that game, we feel like we’re playing the right way,” Giddey said. “The defense is good, we’re locked in on coverages and the scouting [report], and it’s just too up and down. We need to string 48 minutes together and see where we are collectively.

“We’ve had individual stretches and team stretches where we look great, but we have stretches where we look horrible. It’s not one person — it’s a collective effort. And we’ve spoken about it that way, that we have to defend as a team. All of us kind of have to look in the mirror, myself included, and say, ‘What can we do individually to help this group defensively?’ ”

There may not be much. With guard Lonzo Ball sidelined with a sprained right wrist and forward Patrick Williams still inconsistent on both ends, the Bulls don’t really have a stopper, especially against guards and wings. Their best defensive game plan early on this season was to use their up-tempo pace on offense to tire teams out, which was effective at times but not sustainable.

They have learned that during their current four-game losing streak, with the third-worst defensive efficiency in the league over the last three games. Even worse, over those same three games, they have allowed a league-worst average of 38.7 points in the fourth quarter.

Without Ball, the player most needed on defense is guard Ayo Dosunmu, who has shown flashes of being a lockdown defender out of the backcourt. He just needs to do it consistently.

“Shots come and go, but there is a reality of being able to impact the game on a consistent basis on both ends of the floor,” Donovan said. “I do think that Ayo has developed into being a two-way player. I’ve got a lot of confidence in him in terms of our team, our roster, and what he can do. He leans into that. He’s competitive and he takes those challenges on.”